Controlled release of therapeutic agents by means of polymeric materials has existed in various forms for many years. For example, many state-of-the-art medical devices for therapeutic agent delivery have a biostable or biodegradable polymeric coating, which serves as the reservoir for one or more therapeutic agents. Methods of changing the release rate of the therapeutic agent from the coating include changing the therapeutic loading, adding additional polymers to change the hydrophilic/hydrophobic balance of the coating, the use of polymeric barrier layers, and changing the degradation rate (for biodegradable materials). Examples of such medical devices include drug eluting coronary stents commercially available from Boston Scientific (TAXUS), Johnson & Johnson (CYPHER) and others.
Many types of polymeric materials have been used in medical devices. Examples include block copolymers based on poly(butyl methacrylate), poly(vinyl acetate) and polyisobutylene (PIB). It has been found that block copolymers based on PIB have excellent biocompatibility and mechanical properties that make them extremely well suited for use in medical device devices. For instance, block copolymers based on PIB, such as poly(styrene-b-isobutylene-b-styrene), which are typically prepared by living cationic polymerization, have been found to have desirable properties for medical device coatings, particularly those intended to deliver therapeutic agents to the vasculature. These include strength, elasticity, coating conformability, vascular compatibility and biostability, among others.